This invention relates to an optical information recording medium of heat mode write-once type such as CD-R, DVD-R, which is capable of recording or reproducing a data by means of a laser beam, and in particular to an optical information recording medium of write once type, which is improved in light stability.
The CD-R is well known as the means for recording and reproducing video data such as images of character and graphic or audio data such as music. The CD-R now available is formed of an optical disk, which is capable of recording and reproducing with a laser beam of 770 to 830 nm in wavelength.
Recently however, DVD-R (a digital video disk-recordable or a digital versatile disk-recordable) which is capable of recording and reproducing in high density by means of a red laser beam of 620 to 690 nm in wavelength for instance, i.e. a wavelength which is shorter than that of the laser beam employed in the aforementioned CD-R, is now being propagated as new media of the next generation.
As a disk for an optical information recording medium of heat mode write-once type such as CD-R, DVD-R, a pits-forming type optical disk comprising a recording layer containing an organic dye such as cyanine for enabling pits to be formed therein upon irradiation of recording light has been employed. This cyanine dye is advantageous in that it can be applied thereon by a method which is high in productivity such as a spin-coating method, that it is excellent in oxidation resistance, and that it can be locally heated since the heat conductivity thereof is low. In particular, this cyanine dye has been attracting an attention because it exhibits a high absorbancy and reflectivity in a wavelength region of semiconductor laser.
Generally however, the cyanine dye is not sufficiently resistive against sunlight. In order to overcome this problem, various measures have been taken wherein a stabilizing radical or a bonded body consisting of a cyanine dye cation and a quencher anion is generally added to the main component of a cyanine dye exhibiting absorption characteristics to the oscillating wavelength of semiconductor laser so as to transform singlet oxygen generated from sunlight and causing the discoloration of the cyanine dye into triplet oxygen which is more mild in oxidizing effect than the singlet oxygen thereby to improve the light stability of the cyanine-containing layer of CD-R or DVD-R. However, if it is desired to preserve the contents of record for a long period of time as in the case of an encyclopedia for instance, the aforementioned measures are not sufficiently effective, so that there is a strong demand for countermeasures which are capable of preventing a stored data from being photo-degraded for a long period of time.
There has been conventionally proposed the employment of an aluminum salt as a compound which is capable of preventing such a photo-degradation (Japanese Patent Unexamined Publication H7-196588). However, the aluminum salt is poor under the conditions of high temperature and high humidity (for example, 80.degree. C. in temperature and 70% in relative humidity). The employment of nickel dithiol has been also proposed (Japanese Patent Unexamined Publication S57-11090). Although an improvement in some degree of light stability may be expected by the employment of nickel dithiol, the effect thereof is still satisfactory. Particularly when these compounds are employed as a recording medium (DVD-R) for recording or reproducing of data using a short wavelength laser (600 to 630 nm), the photo-degradation-preventing effect thereof is reduced.
Meanwhile, indoaniline derivatives are known to be useful as a dye capable of absorbing near-ultraviolet ray, and hence an information-recording medium employing these compounds as a dye has been also proposed (for example, Japanese Patent Unexamined Publications H4-141493 and H9-175015). According to these documents, the indoaniline derivatives are employed simply as a dye for a recording layer and are not employed as a light stabilizer for improving the light resistance of another kind of dye to be employed in combination with these indoaniline derivatives. In particular, Japanese Patent Unexamined Publication H9-175015 discloses an optical tape medium whose recording layer is improved in light resistance by the employment of an indoaniline derivative. However, the end-use of the indoaniline derivatives described therein is limited to an optical tape medium, i.e. the employment of the indoaniline derivatives for an optical disk is not suggested at all. Furthermore, this document simply teaches that the light resistance of the recording layer can be obtained when these indoaniline derivatives are employed singly as a dye, and hence a technical idea of employing these indoaniline derivatives as a light stabilizer together with another kind of dye for improving the light resistance of said another kind of dye is not suggested at all in this document. Moreover, these indoaniline derivatives are simply exemplified as a dye suited for use for the recording using a laser of 700 to 900 nm in wavelength together with a large number of other kinds of dyes. Thus, nothing is taught or suggested in this prior document about the technical idea of using these indoaniline derivatives as a light stabilizer together with another kind of dye in the recording and reproducing of data using a laser of 600 to 660 nm in wavelength, or using these indoaniline derivatives as a sole dye for improving the light resistance of the recording layer in the recording and reproducing of data using a laser of 600 to 660 nm in wavelength.